The present invention relates to new catalyst systems comprising the product obtained by contacting late transition metal catalyst compounds,and a specific class of alumoxanes: these catalyst systems are particularly active and stable in the homo and copolymerization of olefinic monomers.
Besides metallocene catalysts based on Groups 4 and 5 of the Periodic Table of the Elements (new IUPAC notation), the use of late transition metal complexes for olefin polymerization has been studied and developed in the last years. These complexes exhibit characteristics different from those of well-known metallocenes, constrained-geometry catalysts or traditional Ziegler-Natta catalysts, when used in olefin polymerization. L. K. Johnson et al. (J. Am. Chem. Soc. 117:6414-6415, 1995) describes the use of Ni and Pd complexes with bidentate diimine ligands for xcex1-olefin polymerization; in order to exert a catalytic activity, said complexes are activated with H+(OEt2)2[B(3,5-(CF3)2C6H3)4]xe2x88x92, methylalumoxane (MAO) or Et2AlCl as cocatalysts. These systems have the ability to produce highly branched polymers from ethylene and to copolymerize ethylene with polar monomers.
A class of late transition metal complexes of bidentate xcex1-diimine or xcex2-diimine ligands is disclosed in the international patent application WO 96/23010; said complexes are used in the oligomerization and polymerization of xcex1-olefins, in particular of ethylene, and in the copolymerization of ethylene with polar monomers. The complexes are activated with halo-aluminum alkyl derivatives (such as Et2AlCl, EtAlCl2 and iBu2AlCl), MAO and alkylboronic acid derivatives.
The international patent application WO 98/03559 describes the polymerization of xcex1-olefins or cycloolefins by using one of the above Ni and Pd xcex1-diimine complexes, wherein the cocatalyst is a Lewis acid selected from the group consisting of B(C6F5)3, AlCl3, AlBr3, Al(OTf)3 and compounds of formula (RaRbRcC)Y, wherein Ra-Rc are aryl or substituted aryl groups and Y is a relatively non-coordinating anion. According to this application, compounds commonly used in metallocene activation such as AlMe3, AlEt3, Al(OEt)Et2 and ZnEt2 do not exert any cocatalytic activity with diimine Ni and Pd complexes, unless at least one of the selected cocatalysts is present.
The above-mentioned xcex1-diimine complexes of late transition metals, are also used in polymerization processes at elevated pressure and temperatures. thus obtaining polyethylene products having different molecular weights and branching degrees (see the international patent application WO 97/48737); the catalyst systems are activated with MAO. Bidentate ligands, which are useful in the preparation of Ni complexes active in the polymerization of ethylene, norbornenes and styrenes, are described in the international patent application WO 97/02298; as cocatalysts are used acids of a non coordinating monoanion of formula HX, wherein the preferred anions X were BF4xe2x88x92, PF6xe2x88x92, BAF (i.e. tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) and SbF6xe2x88x92; all the polymerization examples have been carried out in the presence of HBAF(Et2O)2. Further Ni(II) complexes with monoanionic ligands having different structures are described in the international patent application WO 98/30609; said complexes are activated with a Lewis acid cocatalyst, such as BPh3, B(C6F5)3 or BF3, or MAO to polymerize a-olefins, cyclopentene, styrene, norbornene or polar monomers.
The international patent applications WO 98/42664 and WO 98/42665 describe Group 4-10 metal chelates, and in particular Ni or Pd chelates, comprising bidentate ligand compounds of substituted pyrrolaldimine and substituted salicylaldimine. Said chelates are used in catalyst systems for olefin homopolymerization or copolymerization with functionalized xcex1-olefin monomers.
The international patent application WO 98/40374 describes olefin polymerization catalysts containing Group 8-10 metals and bidentate ligands having the following formula: 
wherein R is hydrocarbyl, substituted hydrocarbyl or silyl; A and B are heteroatom connected monoradicals wherein the connected heteroatoms belong to Group 15 or 16, and A and B may be linked by a bridging group. These catalysts optionally contain a Bronsted or Lewis acid as cocatalyst; in the working examples, ethylene oligomerizations or (co)polymerizations are cocatalyzed with MAO. borate compounds, such as HBAr4 (Ar=3,5-bis(trifluoromethyl)phenyl), B(C6F5)3, and aluminum alkyls, such as Et2AlCl.
Recently. Brooke L. Small et al. (J. Am. Chem. Soc. 120:4049-4050, 1998) disclosed Fe(II) and Co(II) catalyst systems incorporating tridentate pyridine duimine ligands having the following general structure: 
wherein R are H, methyl or iso-propyl. The active catalysts, generated by the addition of MAO, are able to convert ethylene to linear high density polyethylene; increasing the steric bulk of the ortho aryl substituents increases molecular weight.
The polymerizations of ethylene and propylene with the above-mentioned complexes of pyridine bis-imine, and more specifically of 2,6-pyridinecarboxaldehyde bis(imines) and 2,6-diacylpyridine bis(imines), are described in the international patent applications WO 98/27124 and WO 98/30612 respectively, wherein the above catalysts are activated by means of the following cocatalysts: methylalumoxane (MAO), boron compounds (such as B(C6F5)3) and aluminum alkyl compounds (such as Et2AlCl and EtAlCl2).
Although the above-described late transition metal catalyst systems are very active in the polymerization of ethylene and may lead to final polymers with interesting structural properties, due to the branching degree, their use is not completely satisfactory, because of the considerable decay of the catalyst activity. In fact, although polymerization activities of these catalysts is quite high in the initial phase of the polymerization, they rapidly decay in the course of the reaction and the deactivation is almost quantitative after few hours. The deactivation mechanism is not known so far. Therefore, these catalysts are not altogether satisfactory if the residence times of the reaction mixture in the reactor are long. This is particularly important in industrial polymerization processes, where it is not possible to operate with short residence times.
As will be demonstrated by the same Applicant in the following, a considerable polymerization activity decay of these catalyst systems occurs in the presence of the cocatalysts tested in the prior art documents described above.
Therefore, it is felt the need of lowering the decay rate and therefore improving the long-term catalytic activity of the above mentioned polymerization catalysts, in order to allow their industrial exploitation.
The Applicant has now unexpectedly found a suitable class of cocatalysts able to activate late transition metal compounds comprising a complex of a metal of group 8, 9, 10 or 11 of the Periodic Table of the Elements (new IUPAC notation) with a bidentate or tridentate ligand; according to the present invention, the catalytic activity in olefin polymerization of the transition metal compounds reported herein can be surprisingly stabilized and therefore enhanced in the long term by adding to these catalysts a specific class of alumoxanes of branched alkylaluminum compounds.
More precisely, the present invention concerns a catalyst system for the polymerization of olefins comprising the product obtainable by contacting the following components:
(A) one or more late transition metal compounds having formula (I) or (II):
LMXpXxe2x80x2sxe2x80x83xe2x80x83(I)
LMAxe2x80x83xe2x80x83(II)
xe2x80x83wherein M is a metal belonging to Group 8, 9, 10 or 11 of the Periodic Table of the Elements (new IUPAC notation);
L is a bidentate or tridentate ligand of formula (III): 
xe2x80x83wherein:
B is a C1-C50 bridging group linking E1 and E2, optionally containing one or more atoms belonging to Groups 13-17 of the Periodic Table;
E1 and E2, the same or different from each other, are elements belonging to Group 15 or 16 of the Periodic Table and are bonded to said metal M;
the substituents R1, the same or different from each other, are selected from the group consisting of hydrogen, linear or branched, saturated or unsaturated C1-C20 alkyl, C1-C20 alkyliden, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals, optionally containing one or more atoms belonging to groups 13-17 of the
Periodic Table of the Elements (such as B, Al, Si, Ge, N, P, O, S, F and Cl atoms); or two adjacent R1 substituents form a saturated, unsaturated or aromatic C4-C8 ring, having from 4 to 20 carbon atoms;
m and n are independently 0, 1 or 2, depending on the valence of E1 and E2, so to satisfy the valence number of E1 and E2; q is the charge of the bidentate or tridentate ligand so that the oxidation state of MXpXxe2x80x2s or MA is satisfied, and the compound (I) or (II) is overall neutral;
the substituents X, the same or different from each other, are monoanionic sigma ligands selected from the group consisting of hydrogen, halogen, xe2x80x94R, xe2x80x94OR, xe2x80x94OSO2CF3, xe2x80x94OCOR, xe2x80x94SR, xe2x80x94NR2 and xe2x80x94PR2 groups, wherein the R substituents are linear or branched, saturated or unsaturated, C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl radicals, optionally containing one or more atoms belonging to groups 13-17 of the Periodic Table of the Elements (new IUPAC notation), such as B, N, P, Al, Si, Ge, O, S and F atoms; or two X groups form a metallacycle ring containing from 3 to 20 carbon atoms; the substituents X are preferably the same;
Xxe2x80x2 is a coordinating ligand selected from mono-olefins and neutral Lewis bases wherein the coordinating atom is N, P, O or S;
p is an integer ranging from 0 to 3, so that the final compound (I) or (II) is overall neutral;
s ranges from 0 to 3; A is a xcfx80-allyl or a xcfx80-benzyl group; and
(B) the reaction product of water with one or more organometallic aluminum compounds of formula (IV):
Al(CH2xe2x80x94CR3R4R5)xR6yHzxe2x80x83xe2x80x83(IV)
xe2x80x83wherein, in any (CH2xe2x80x94CR3R4R5) groups, the same or different from each other, R3 is a linear or branched, saturated or unsaturated C1-C20 alkyl, C3-C20 cycloalkyl or C7-C20 alkylaryl radical, optionally containing one or more Si or Ge atoms; R4 is a saturated or unsaturated C3-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl radical, optionally containing one or more Si or Ge atoms, said radical being different from a straight alkyl or alkenyl group; or R3 and R4 form together a C4-C6 ring; R5 is hydrogen or a linear or branched, saturated or unsaturated C1-C20 alkyl, C6-C20 aryl, C7-C20 alkylaryl or arylalkyl radical, optionally containing one or more Si or Ge atoms;
R6 is a linear or branched, saturated or unsaturated C1-C20 alkyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl radical;
x is an integer ranging from 1 to 3; z is 0 or 1; and y is 3xe2x88x92xxe2x88x92z. the molar ratio between said organometallic aluminum compound and water being comprised between 0.5:1 and 100:1.
The present invention further provides a process for the (co)polymerization of olefins comprising the reaction of polymerization of one or more olefinic monomers in the presence of a catalyst system as reported above.